The invention relates to infusion pumps suitable, e.g., for continuously delivering a drug to a patient at a rate determined by one of a plurality of predetermined speeds of movement of a plunger in the pump syringe barrel. The invention also relates to discretely adjustable speed conversion means which may be employed, e.g., for the selection of one of the plunger speeds of an infusion pump, and thereby the rate of metering of the drug to a patient.
Conventional types of infusion pumps employ a syringe barrel with a movable plunger for administering a constant flow of a drug to a patient, with the position of the plunger being adjusted by a mechanized dose rate control that is powered by a prime mover, typically a variable-speed motor. In general, the dose rate control may comprise a gear box or other suitable speed conversion means for reducing the speed of the variable-speed motor to one of a plurality of relatively slow settings appropriate for driving the plunger in the syringe barrel.
The syringe barrel of conventional infusion pumps is made out of either glass or plastic. The glass device is extremely accurate in its dimensions, and thereby in the accuracy of the setting of the dose rate; however, it is necessary to thoroughly wash and sterilize such glass syringes after each use. Any residual contaminants, even though sterile, can be fatal when the glass syringe is re-used for another patient.
While such dangers may be eliminated by using a standard, "throw-away" pre-sterilized syringe, other problems crop up in their place. For example, such pre-sterilized syringes are generally manufactured under poor quality control, so that their interior dimensions are not accurate. Such inaccuracies, in turn, lead both to irregularities in the dose rate setting and to poor fit of the plunger inside the barrel. Under the latter condition, and particularly during the very slow rates of flow of the drug which are ordinarily required, the medication may escape from the barrel via the gap between the plunger and the barrel, rather than being infused into the patient as desired.
An additional disadvantage of such existing infusion pumps is their relatively coarse control of the setting of dose rate. For example, where the rate controller is a mechanized gear box whose rate may be set by a suitable rate control knob, it is not uncommon to observe a 50 - 100% change in dose rate between a pair of adjacent settings of the rate control knob. Moreover, when, as is conventional, the rate controlling gear boxes are driven by servo-controlled variable-speed electric motors, unpredictable changes in line voltage and the like may result in dangerous and sometimes fatal changes in the ultimate dose rate from the pump.